Distillation is an operation whereby the vaporization of a liquid mixture yields a vapor phase containing more than one component, and it is desired to recover one or more of these components in a pure state. This is distinct from evaporation which is not discussed in this section.
Distillation as such is a major unit operation in the chemical processing industry for the purification or separation of liquid mixtures. Other principle unit operations of separation are evaporation and extraction. Distillation and evaporation are often considered simultaneously because of their use of heat to achieve separation.
Distillation in practical operations can be effected on either a continuous or a batch mode of operation. A simple reboiler and condenser arrangement with an unpacked column provides one theoretical stage of separation. This is suitable for mixtures containing a volatile solvent with non volatile impurities. Columns which are packed or fitted with trays provide several theoretical stages of separation. Such arrangements are valuable for liquid mixtures in which both components have similar relative volatilities.
Although the effectiveness of the distillation operation is dependent upon such theoretical considerations as the relative volatility of the components. In practice, the design of a distillation unit is extremely important.
Borosilicate glass equipment has been used successfully for many years in the field of distillation operations. Many units have been installed using the distillation operation, especially for the recovery of solvents. However, details on this particular aspect are not covered in this section.
Glass distillation columns are normally filled with packing materials made of borosilicate glass, but other packing materials can also be supplied. Cooling arrangements for the distillate can use either shell and tube or coil type heat exchangers. Specific advice on both the optimum packing material and the method of operation can be given by our Chemical Engineers.
Glass columns can vary in diameter from 80 to 1000 mm and columns have been erected to heights of up to 30 meters. Glass distillation units can operate at atmospheric pressure or at high vacuum, using special low pressure loss packings.
Distillation Unit with Coil Type Condenser
The distillation arrangement with descending type coil condenser is one of the simplest types of condenser arrangements and includes a total condenser, product cooler and vent condenser.
By combining glass process plant equipment with other materials, we can offer well proven units in a a variety of sizes. Glass reaction vessels are available up to 400 liters in capacity and, where larger units are required, glass-lined and stainless steel vessels are commonly used. Coil type condensers have surface areas from 0.2m2 up to 12m2. Using cooling water at 20°C these condensers have heat transfer coefficients of up to 250 kcal/m2/h°C.
This up and over type distillation unit is suitable for use under vacuum conditions and is an ideal facility for reactions involving total reflux.
Distillation Unit with Shell and Tube Condenser
This distillation arrangement is virtually identical to the previous arrangement. The main difference is that shell and tube heat exchangers have been incorporated instead of coil condensers. Shell and tube type condensers have surface areas from 3m2 to 26m2. Using cooling water at 20°C these condensers have heat transfer coefficients of up to 900 kcal/m2h°C.
The most important feature of this arrangement is the low installation height.
Distillation Unit with Phase Separation
This distillation arrangement illustrates the use of a horizontal separation vessel in a distillation unit.
In this arrangement, the light phase flows back into the boiler as the reflux and the heavy phase flows through a condenser into the receiver as the product. It is easy to reverse the system to enable the heavy phase to become the reflux.
The arrangement can be operated under vacuum and the illustration shows suitable receivers.
Batch Distillation Column
Batch distillation columns always have a reboiler vessel sized to accommodate the entire batch of the material to be distilled.
For small batch operations, glass vessels up to 200 liters in capacity are normally used and, where larger batches are required, glass-lined or stainless steel vessels are available.
Glass distillation columns are available from 80 to 100 mm in nominal bore and are ideal for operation under both atmospheric and vacuum conditions.
Columns of this nature are operated under conditions of either fixed reflux ratio or variable reflux ratio. This illustration shows a batch distillation column which includes a total condenser, product cooler and the facility for providing reflux at the top of the column by means of the control valve. Reflux timers are also available.
Continuous Distillation Column
Continuous distillation columns always have this same basic construction. For the reboiler, a circulatory evaporator is often selected and, as pre-heaters, our HEB type boilers are ideal.
The main point to consider in continuous distillation columns is the automatic control system. Many standard systems are available and, in this illustration an electro-magnetically controlled reflux divider with timer is shown.
Glass distillation columns are available from 80 to 1000 mm in nominal bore and ideal for operation under both atmospheric and vacuum conditions.